Throughout this application various patents and published patent applications are referred to by an identifying citation. The disclosures of the patents and published patent applications referred to in this application are hereby incorporated by reference into the present disclosure to more fully describe the state of the art to which this invention pertains.
An electroactive material that has been fabricated into a structure for use in a battery is referred to as an electrode. Of a pair of electrodes used in a battery, herein referred to as a chemical source of electrical energy, the electrode on the side having a higher electrochemical potential is referred to as the positive electrode, or the cathode, while the electrode on the side having a lower electrochemical potential is referred to as the negative electrode, or the anode.
An electrochemically active material used in the cathode or positive electrode is referred to hereinafter as a cathode active material. An electrochemically active material used in the anode or negative electrode is hereinafter referred to as an anode active material. Multi-component compositions possessing electrochemical activity and comprising an electrochemically active material and optional electron conductive additive and binder, as well as other optional additives, are referred to hereinafter as electrode compositions. A chemical source of electrical energy or battery comprising a cathode with the cathode active material in an oxidized state and an anode with the anode active material in a reduced state is referred to as being in a charged state. Accordingly, a chemical source of electrical energy comprising a cathode with the cathode active material in a reduced state, and an anode with the anode active material in an oxidized state, is referred to as being in a discharged state.
The value of the free space of voids in the electrode (cathode or anode) composition expressed in the percentages from the overall volume of the electrode (cathode or anode) composition layer is hereinafter referred to as a porosity of the electrode (cathode or anode) active layer.
Since batteries continue to evolve, and particularly as lithium batteries become more widely accepted for a variety of uses, the need for safe, long lasting, high energy density, and lightweight batteries becomes more important. There has been considerable interest in recent years in developing high energy density cathode active materials and alkali metals as anode active materials for high energy primary and secondary batteries.
To achieve high capacity in chemical sources of electrical energy or batteries, it is desirable to have a high quantity or loading of electroactive material in the cathode or anode active layer. For example, the volume of the cathode active layer in an AA size battery is typically about 2 cm3. If the specific capacity of the electroactive material is very high, for example 1000 mAh/g, the amount or volumetric density of the electroactive material in the cathode active layer would need to be at least 500 mg/cm3 in order to have the 1 g of cathode active material in the AA size battery necessary provide a capacity of 1000 mAh. If the volumetric density of electroactive material in the cathode active layer can be increased to higher levels, such as greater than 900 mg/cm3, the capacity of the battery may be proportionately increased to higher levels if the specific capacity of the electroactive material does not decrease significantly when the cathode active layer becomes denser and less porous.
There are a wide variety of electroactive materials that may be utilized in the cathode active layers of chemical sources of electrical energy. For example, a number of these are described in U.S. Pat. No. 5,919,587 to Mukherjee et al. These electroactive materials vary widely in their specific densities (g/cm3) and in their specific capacities (mAh/g) so the desired volumetric densities in mg/cm3 of the electroactive material in the cathode active layer correspondingly vary over a wide range. Lithium and sulphur are highly desirable as the electrochemically active materials for the anode and cathode, respectively, of chemical sources of electrical energy because they provide nearly the highest energy density possible on a weight or volume basis of any of the known combinations of active materials. To obtain high energy densities, the lithium may be present as the pure metal, in an alloy, or in an intercalated form, and the sulphur may be present as elemental sulphur or as a component in an organic or inorganic material with high sulphur content, preferably above 75 weight percent sulphur. For example, in combination with a lithium anode, elemental sulphur has a specific capacity of 1680 mAh/g. This high specific capacity is particularly desirable for applications, such as portable electronic devices and electric vehicles, where low weight of the battery is important.
Practical battery cells comprising the electroactive cathode and anode materials also typically contain other non-electroactive materials such as a container, current collectors, separator and electrolyte, in addition to polymeric binders, electrically conductive additives, and other additives in the electrodes. The electrolyte is typically an aqueous or nonaqueous liquid, gel or solid material containing dissolved salts or ionic compounds with good ionic conductance, but with poor electronic conductivity. All of these additional non-electroactive components are typically utilized to make the battery perform efficiently, but they also contribute to a reduction of the gravimetric and volumetric energy density of the cell. It is, therefore, desirable to keep the quantities of these non-electroactive materials to a minimum so as to maximize the amount of electroactive material in the battery cell.
To achieve the highest possible volumetric density of the electroactive material in the cathode or anode active layer, it is desirable to maximize the weight percent for electroactive materials in the cathode or anode active layer, for example up to 65-85 weight percent, and to maintain the porosity or air voids in the cathode or anode active layer as low as possible, for example, in the range of 30 to 60 volume percent. In particular, the porosity of the cathode active layer must be kept low because higher porosities, such as, for example, 70 to 85 volume percent, do not provide enough electroactive material to obtain very high cell capacities.
Electroactive materials are typically electrically non-conducting or insulative and are generally not microporous. To overcome the insulative properties of electroactive materials, certain amounts of electrically conductive fillers, such as conductive carbons, are typically added to the cathode active layer. Typically, the electrically conductive fillers are present in amounts of about 5 to 40% by weight of the cathode active layer. For example, U.S. Pat. No. 4,303,748 to Armand et al. describes solid composite cathodes containing an ionically conductive polymer electrolyte together with elemental sulphur, transition metal salts, or other cathode active materials for use with lithium or other anode active materials. U.S. Pat. No. 3,639,174 to Kegelman describes solid composite cathodes comprising elemental sulphur and a particulate electrical conductor. U.S. Pat. No. 5,460,905 to Skotheim describes the use of p-doped conjugated polymers, together with an effective amount of conductive carbon pigments, for the transport of electrons in cathodes. U.S. Pat. Nos. 5,529,860 and 6,117,590, both to Skotheim et al., describe the use of conductive carbons and graphites, conductive polymers, and metal fibres, powders, and flakes with electroactive materials.
It would be advantageous significantly to increase the volumetric densities of cathode or anode active layers comprising electroactive materials without sacrificing the high specific capacity of these materials, i.e., without reducing the desired high electrochemical utilization, such as, for example, greater than 50% utilization, during cycling of the cells. Particularly as the thickness of the cathode or anode active layer is increased, it becomes progressively more difficult to achieve the electrical conductivity and the microporosity needed for highly efficient electrochemical utilization of the active materials.
Some improvement in the methods of forming solid composite cathodes with cathode active layers which comprise an electroactive sulphur-containing material and an electrically conductive material are described in U.S. Pat. No. 6,302,928 to Xu et al. This patent refers to a method of forming electric current producing cells, wherein the electroactive sulphur-containing material is heated to a temperature above its melting point to form a melt layer and then is resolidified to form a cathode active layer. This method is not free of significant drawbacks, since obtaining a high density of sulphur-containing active material reduces its porosity and hence the availability of the active material. Besides, this method is not applicable to the other active cathode materials that have a melting temperature too high for producing cathodes in the way described in the said U.S. Pat. No. 6,302,928.
Another method to increase the volumetric density of the cathode active layer is by compressing or calendering the layer to a reduced thickness. It would be very advantageous to be able to compress or calender the cathode active layer to a 20% or greater reduction in thickness without sacrificing the desired high electrochemical utilization of the electroactive sulphur-containing materials. This is difficult to achieve when high levels of non-electroactive materials are present in the cathode active layer, particularly when polymeric binders are used, such that the electrochemical utilization, as expressed in the specific capacity of the electroactive material in the cell, is typically significantly reduced when the cathode active layer is significantly reduced in thickness by compressing or calendering of the whole cathode layer. On the other hand, significantly reducing the levels of the non-electroactive materials in the cathode active layer, particularly those materials with binding properties, greatly reduces the mechanical integrity and cohesive and adhesive properties of the cathode active layer.
As mentioned above, the porous electrodes of the chemical sources of electrical energy are usually multi-component solids, comprising an electrode depolarizer (the liquid or hard active substance), an electron conducting additive (the substance providing transport of electrons to the depolarizer), and a binder (the substance ensuring the mechanical strength of the electrodes). The electrodes may also include auxiliary components improving the mechanical and electrochemical properties of the electrode materials. The electrode pores are filled up with electrolyte (a liquid or a hard substance possessing ion conductivity). An electrochemical reaction occurs at the three-phase interface of the depolarizer, electron conductor and ion conductor. The electrochemical reaction efficiency is determined by the electrochemical properties of the depolarizer and by the ion and electron conductance of the electrode. The ion conductivity is usually much (1 to 3 orders of magnitude) lower than the electron conductivity of the electrode. Thus, the value of the electrochemical overvoltage that determines the speed of the electrochemical reaction is in turn defined by the ion resistance of the electrode. The maximum values of electrochemical overvoltage and the maximum speed of the electrochemical reaction are reached at the face side of the electrode (the surface turned towards the opposite electrode of the chemical source of electric energy). The minimal values are reached at the rear side of the electrode (the surface turned to the current collector of the electrode). As a result, the depolarizer is consumed to a greater extent at the layers close to the face side of the electrode and to a lesser extent at the layers of the electrode close to its rear side. The gradient of the overvoltage and hence the gradient of the current density over the electrode thickness grows with the increase of the overall current density of the chemical sources of electric energy. This leads to an increase in the heterogeneity of the reaction distribution over the electrode thickness and in extreme cases to a full ousting of the electrochemical reaction to the surface of the electrode.
The electrochemical overvoltage in the cathodes of the chemical sources of electrical energy has a diffusion (concentration) nature. The current density controlled by diffusion is determined by the ratio of the electrolyte volume inside the pores and the area of the working surface of the electrode. In particular, the current density is reduced as this ratio is decreased.
In particular, the porosity value is crucial for chemical sources of electrical energy with soluble depolarizers, examples of which are the systems: Li—SO2; Li—SOCl2; Li—S.
Despite the various approaches proposed for the fabrication of high energy density chemical sources of electrical energy comprising various electroactive materials, there remains a need for improved solid composite cathodes and anodes comprising an active layer which has a combination of high electrochemical utilization and a high volumetric density of the electroactive material, while retaining or improving the desirable properties of electrical conductivity, mechanical strength, cohesive strength, and adhesion to the adjacent layers in the porous solid composite electrodes utilizing electroactive materials.
On the one hand, an increase in the density of the electrodes (decrease in porosity) produces an increase of the energy density of chemical sources of electrical energy. On the other hand, a decrease in porosity reduces the ion conductivity of the electrodes and hence makes the electrochemical reaction conditions and the utilization of active materials worse.